Magnet structure for loud-speakers



Dec. 5, 1950 s. E. LEVY 2,532,413

MAGNET STRUCTURE FOR LOUDSPEAKERS Filed Dec. 51, 1946 2 Sheets-Sheet l INVENTOR SIDNEY E. LEVY olwlgflg ATTORNEYS Dec. 5, 1950 s. E. LEVY MAGNET STRUCTURE FOR LOUDSPEAKERS 2 Sheets-Sheet 2 Filed Dec. 31, 1946 INVEN'TOR SIDNEY E. LEVY sv g a m Patented Dec. 5, 1 950 MAGNET STRUCTURE FOR LOUD-SPEAKERS Sidney E. Levy, Brooklyn, N. Y. assignor to University Loudspeakers, 1110., New York, N. Y., a corporation of New York Application December 31, 1946, Serial No. 719,430

1 Claim. 1

The present invention relates to magnet structures, and particularly to magnet structures of the type in which the center pole piece and bottom keeper are cast as part of the magnet. A cross sectional view of such a magnet has an E-shaped appearance and these magnets are particularly utilized in loudspeaker construction. In the past these magnets have been difficult to produce unless the magnet had a curved outer surface, since, in casting, stresses were set up which tended to produce cracks as the material cooled. The curved outer surface mentioned presented a major disadvantage in the assembling of the speaker, and additionally, required the use of a magnetizing coil which could not be removed without great difiiculty. By my pres ent invention I have made it possible to utilize a pot type magnet which is E-shaped in cross-section, thus having plane outer surfaces which can readily be incorporated into a speaker construction.

It is an object of my invention to provide a magnet structure of the pot type, the cross section of which is of E-shape.

It is another object of my invention to produce such a magnet having slots therein extending the full height of the arms of the E.

It is another object of my invention to provide a magnet of the type mentioned in which cracking during fabrication is substantially eliminated.

It is a further object of my invention to provide a magnet of the type described having the slots mentioned, whereby the magnetizing windings may be readily removed subsequent to the final placing of the keeper upon the magnet.

Other objects and features of the invention will appear when the following description is considered in connection with the appended drawings, in which- Figure 1 is a top plan view of the magnet structure of my invention with the air gap keepers in position;

Figure 2 is a view similar to Figure 1 but showing a magnet with the keeper (or cover plate) removed;

Figure 3 is a ertical cross sectional view of the magnet structure of Figure 1 taken on the plane of the line 3--3 of Figure 1;

Figure 4 is a diagrammatic showing illustrating one mode of winding a magnetizing winding upon the magnet of Figures 1 and 2 in such manner that the winding may readily be removed after the magnet has been magnetized; and

Figure 5 is a view similar to Figure 4 but showing a difierent mode of winding a magnetizing winding upon the magnet.

Referring now to the drawings, and particularly to Figure 3, the magnet comprises an inner pole piece [0 and an outer annular pole piece H, these pole pieces being, of course, integral and the shape being procured by casting in a suitable mold. The mold is provided with upstanding members in order to produce a magnet having a true E-shaped cross section with a plurality of slots in its outer rim without setting up such stresses as to cause cracking of the casting dur ing cooling. Additionally, the slots thus formed provide an advantageous mode of assembly of the magnet into a loudspeaker structure, since bolts may be passed through the slots with their heads extending across the bottoms of the slots and screwed into some other portion of the loudspeaker structure. The outer edge of the inner pole piece ID is chamfered, as indicated at I3, in order to provide a space for the spacer ring l4 and to avoid magnetic leakage to the outer keeper. This chamfering is at an angle of not more than 45 degrees with the horizontal, and thus there is no effective diminution of the flux across the magnetic gap I5 existing between the cooperating inner and outer keepers l6 and 51.

Since in any loudspeaker it is desirable that the axial depth be as much reduced as is possible, the structure presented possesses a great advantage. In the particular structure mentioned, the ratio of the axial length or thickness of the magnet to its diametric dimension may be from 3:1 to 5:1, and this is due primarily to the fact that the magnet may be cast with sharp edges without fear of cracking, due to the use of the slots, as has been mentioned. Were it not for this provision, the axial length would necessarily be increased since the outer surface, in order to assure a reasonable rate of rejects, would necessarily be curved to avoid the cracking of the castings as mentioned above.

In addition to the advantages already stated for the magnet of my invention, reference to Figure 4 and 5 will show that by winding the magnetizing winding through the slots and about the intervening portions of the magnet, there is provided a means for magnetizing the casting which does not necessitate leaving the winding entirely within the internal groove of the magnet. This is advantageous since the volume of air behind the diaphragm must be of a definite value. This is clearly shown in both Fig- It will be noted that in one of these forms the winding is wrapped about ,be understood that the windings are wound or placed in position on the magnet. before the assemblage of the keepers to the magnet, and that the magnetizin is done subsequent to the placing of the keepers (or cover plate) I6 and H on the magnet structure. With the old arrangement, lacking the slots I2, it is necessary to leave the winding in position after the magnetization is completed since there is no way available for removing it without also removing the keepers and. thus breaking the magnetic circuit causing a reduction in the magnetomotive force of the assembly. As a result, there are variations in the volume of air space behind the speaker diaphragm which may detrimentally afiect speaker operation.

In'order to provide for a controlled movement of air the face of the center pole piece portion of theemagnet may have one or more slots I9 to facilitate escapement of the rear wave pressure exerted by the diaphragm. This back pressure is partly released as follows: through the center hole 28 in the machined keeper or cover I8, through the groove or slot I9 in the center pole I and from here expended in two directions into the large interior area of the ma net. The cross. sectional area of slot I9 may be so dimensioned as to serve as an acoustic resistance to provide some discrete compensation in the frequency response in addition to its service for back pressure release.

While I have described a preferred form of my invention, it will be understood that alternate forms may be devised without departin from the spirit of the invention. Consequently, I wish to be limited not by the foregoing description, which was given solely for purposes of illustration, but only by the appended claim.

What is claimed is:

A magnet assembly for a loudspeaker or the like comprising a generally cylindrical block of magnetized metal having the cross-sectional shape of an E forming an outer rim and inner cylindrical portion, the outer rim of said magnet having one magnetic polarity and the inner cylindrical portion the other magnetic polarity, a cover plate assembly comprising an outer annular plate and an inner circular plate fixed together, said assembly being mounted on said magnet and forming an annular magnetic gap, a plurality of slots extendin radially inwardly through the rim portion and annular groove of the magnet, holes in said cover plate mating with said slots and forming means to mount said magnet assembly, an opening in said inner circular plate and a. groove in the face of said inner cylindrical portion of the magnet, said groove communicating with said opening.

SIDNEY E. LEVY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS- 

